1. Field of the Invention
This invention relates generally to a Mach-Zehnder modulator for modulating an optical beam with an RF signal and, more particularly, to a coupled quantum well Mach-Zehnder modulator for modulating an optical beam with an RF signal employing in-phase RF driven push-pull electrodes.
2. Discussion of the Related Art
Optical modulators are often used to impress an electrical signal onto an optical carrier beam. Optical modulators of this type that operate with low modulations voltages are needed in analog photonic link applications to improve link gain and noise figure. For digital applications, the optical beam is modulated by turning the light beam on and off to differentiate zero bits and one bits. In analog applications, the optical beam is modulated so that the intensity of the light beam is proportional to the amplitude of the RF signal. The present invention deals with analog modulation applications, and has particular application for transmitting RF signals to a satellite and to provide optical correlation for signal processing.
One class of modulators that operate at low modulation voltages are semiconductor modulators that employ PIN semiconductor devices where the optical wave propagates down an active region defined by the intrinsic layer in the device. An RF signal is applied to electrodes in contact with the P and N layers of the device to provide the modulation voltage across the intrinsic layer, where the electrodes define an RF transmission line. The RF signal alters the index of refraction of the intrinsic layer, which acts to change the speed of the light beam propagating therethrough, so as to provide the modulation of the light beam based on the amplitude of the RF signal.
One type of optical modulator that employs PIN semiconductor devices is a Mach-Zehnder modulator (MZM). An MZM employs a Mach-Zehnder interferometer that splits the optical beam being modulated into first and second waveguide arms, where one of the waveguide arms extends through the intrinsic layer of a first PIN semiconductor device and the other waveguide arm extends through the intrinsic layer of a second PIN semiconductor device. The optical beams propagating through the two arms are then recombined in a recombiner, where the two beams constructively or destructively interfere depending on their relative phases. Thus, the manner in which the RF signal alters the index of refraction in the PIN device determines the intensity of the output beam.
Separate electrodes are connected to the N and P layers of the first and second PIN devices to modulate the optical beam propagating therethrough. Conventional semiconductor PIN MZMs typically only use single electrode drives where the second arm of the interferometer is a passive optical waveguide. In other types of MZMs, such as lithium niobate or polymer MZMs, the advantages of employing push-pull modes of operation, known to those killed in the art, have been demonstrated. In the push-pull configuration, typically the RF power delivered to the first arm is applied 180 degrees out-of-phase with the RF power delivered to the second arm. In other words, by applying opposite RF signals to the PIN devices in the two arms, a doubling of the effective RF signal is provided, where the RF signal has an opposite effect on the index of refraction in the intrinsic layers of the device. Thus, less RF power needs to be employed for a particular change in index of refraction. In this case, the change in phase in the two arms are equal and opposite which has the effect of increasing the amplitude modulation of the optical field and eliminating chirp.
The known push-pull MZMs are typically implemented by splitting the input RF power using a 180 degree hybrid power splitter, and then applying the two RF outputs to the two arms of the modulator. However, this technique has the disadvantage of requiring broadband power splitters that must maintain strict phase control over the frequency band of interest. Because these devices use power splitting, the actual improvement in the reduction of the modulation voltage is only a factor of √{square root over (2)} over that of the single arm drive modulators.
The Vπ of a Mach-Zehnder modulator can be decreased by a factor of √{square root over (2)} or by a factor of x2, depending on whether an RF power split is required, by driving both arms of the modulator with the RF driving voltage. This improvement in Vπ does not decrease modulator bandwidth. To date, push-pull electrodes on PIN semiconductor modulators have been driven 180 degrees out-of-phase, so that the retardation of the optical phase front is increased in one arm, while being decreased in the other arm. Lithium Niobate modulators can employ a co-planer electrode structure which enables a single electrode push-pull operation, but PIN semiconductor modulators have not been realized with this type of co-planer electrode. When a conventional RF source is used, driving the electrodes 180 degrees out-of-phase is not simple at high frequencies (GHz) since the methods available for producing the 180 degrees out-of-phase RF signal are not necessarily broadband. Dual RF output drivers are available, typically using MMIC technology, which do produce the two 180 degree out-of-phase signals needed to drive a conventional dual electrode MZM push-pull.
Most Mach-Zehnder modulator designs strive for the greatest amount of the electro-refraction, since digital modulators are designed to turn an optical signal on and off. The performance of analog RF modulators, on the other hand, depend on the slope of the electro-refraction d(Δn)/dV in the intrinsic layer. In typical quantum well modulators, the electro-refractive effect is governed by the quantum confined Stark effect (QCSE), in other words, the electro-refraction is quadratic with voltage. Thus, a structure which has a large amount of electro-refraction (good for digital applications), is very similar to the quantum well structure which produces a high slope of the electro-refraction.
What is needed is a PIN Mach-Zehnder modulator that provides a push-pull operation so as to limit the RF voltages, and does not require that applied to the arms of the modulator 180° out of phase with each other. It is therefore an object of the present invention to provide such a Mach-Zehnder modulator.